Hydrogen is increasingly important in processing hydrocarbons and the demand for refinery hydrogen has increased in recent years. By far the most practical way to produce hydrogen on a large scale (e.g. suitable for feed to a hydrotreater or hydrocracker) is the steam reforming reaction sequence where steam and a hydrocarbon are reacted. In the case of a natural gas feed, two molecules of hydrogen are released for every molecule of methane and one molecule for each molecule of steam    CH4+H2O→3H2+CO (steam reforming)    CO+H2O→H2+CO2 (water-gas shift)    CH4+2H2O→4H2+CO2 (overall)
This reaction scheme points, however to one obvious dilemma—the production of carbon dioxide, which in mass terms is 2.51 times more than the hydrogen product being produced. Furthermore, missing from this scheme is the highly endothermic nature of the steam reforming reaction (+206 kJ/mol) that despite the fact it is catalysed by a base-metal catalyst (usually nickel on a ceramic carrier) requires a significant input of heat and high temperatures generally in excess of 800° C. This is generally achieved using a large fired heater with a multitude of process tubes containing the catalyst. The fuel burnt in the steam reformer also represents another source of carbon dioxide.
Processes for generating hydrogen on an industrial scale are known. Known processes typically comprise steam reforming a hydrocarbon such as naphtha or natural gas that has been purified to remove sulphur and/or chloride compounds, subjecting the resulting crude synthesis gas comprising hydrogen, carbon oxides and steam to a high temperature water gas shift step to increase the hydrogen content, cooling the shifted gas and removing the condensate and then separating the carbon dioxide from the de-watered shifted gas mixture to generate the hydrogen.
A modern-day hydrogen plant is not as efficient as the equations above suggest. Only approximately 90% of the hydrogen ends-up in the product stream with much of the remainder being burnt as fuel. Though the unconverted methane is also burnt, it is wasteful as natural gas is a more expensive reformer “fuel” than say a typical refinery fuel. Better would be to have more of the methane converted to hydrogen product and burn more refinery fuel instead. Finally, the reduction in carbon dioxide released to the atmosphere without compromising the cost of hydrogen production needs consideration
Pre-reforming the hydrocarbon upstream of the steam reformer by passing the purified hydrocarbon and steam adiabatically through a bed of steam reforming catalyst offers some advantages, but the process efficiency may be improved further.